A nonaqueous electrolyte secondary battery, typically a lithium ion secondary battery, has a high energy density, and thus lets a large current flow and generates heat intensely in a case where a breakage of the battery, for example, penetration of a foreign body (typically, a nail penetration test), has caused an internal short circuit.
There is thus a demand that a nonaqueous electrolyte secondary battery should have the function of preventing more than a certain level of heat generation. A known nonaqueous electrolyte secondary battery having such a function is a battery including a separator having a shutdown function, which is a function of blocking passage of ions between the cathode and the anode with use of a separator in a case where there has been abnormal heat generation. This function can prevent further heat generation.
An example separator having a shutdown function is a porous film made of a material that is meltable by abnormal heat generation. A battery including such a separator is arranged such that abnormal heat generation melts the porous film to make the porous film non-porous and thus block passage of ions, thereby preventing further heat generation.
An example separator having such a shutdown function is a porous film containing a polyolefin as a main component. A polyolefin porous film as a separator is, in a case where there has been abnormal heat generation in the battery, melted at a temperature of approximately 80° C. to 180° C. and made non-porous to block passage of ions, thereby preventing further heat generation. However, in a case where, for example, there has been intense heat generation, a polyolefin porous film as a separator may, for example, contract or break, and let the cathode and the anode come into direct contact with each other, thereby causing a short circuit. A polyolefin porous film as a separator, as described above, has insufficient shape stability and does not necessarily prevent abnormal heat generation resulting from a short circuit.
There have been proposed some nonaqueous electrolyte secondary battery separators that are excellent in shape stability at high temperatures. One of such nonaqueous electrolyte secondary battery separators proposed is a nonaqueous electrolyte secondary battery separator including a laminated porous film that includes a laminate of (i) a heat-resistant layer containing fine particles as a filler and (ii) a porous film containing a polyolefin as a main component (see, for example, Patent Literatures 1 to 3). A nonaqueous electrolyte secondary battery separator including such a laminated porous film is excellent in shape stability and can prevent abnormal heat generation resulting from a short circuit.